1. Field of the Invention
The invention relates to crosslinkable compositions of silane-crosslinkable prepolymers, methods for producing them, and their use as adhesives and sealants, more particularly as adhesives with high tensile shear strength for bonding applications including wood bonding.
2. Description of the Related Art
Among the systems known for implementing bonds to wood are wood glues, formulated typically on the basis of polyvinyl acetate dispersions. While these glues do exhibit effective adhesion to wood, their setting rate, i.e., the time which elapses until a load-bearing bond has formed, is nevertheless very long, meaning that long-lasting mechanical fixing of the workpieces to be bonded is generally unavoidable. Furthermore, the use of this type of adhesive presents problems if the bond is exposed to moisture, since the wood glues typically have a limited resistance toward water. In the case of wooden constructions which receive high loads, where requirements in relation to the mechanical strength of the components are exacting and where the bond strength is still to be high enough even after many years under effects of weathering, wood glues of this kind are usually not suitable.
Here, typically, isocyanate-crosslinking PU adhesives are employed. These adhesives usually comprise aromatic polyisocyanates. Systems of this kind cure by reaction of the isocyanate groups with (atmospheric) moisture. Since PU adhesives cure via a chemical crosslinking reaction and are also able to attach chemically to a wood substrate, they exhibit significantly better mechanical properties and are also substantially more resistant toward external (weathering) effects such as moisture or direct water contact.
The general performance of adhesives is ascertained by their compliance with standards, such as, for example, DIN EN 204, durability class D1-D4. These standards can generally be met by isocyanate-crosslinking adhesives.
Nevertheless, even some isocyanate-crosslinking adhesives possess massive disadvantages inherent in the system. For example, one-component PU adhesive systems generally possess no more than moderate cure rates. It is true that the isocyanate crosslinking can in principle be accelerated sharply by catalysis. However, since such catalysis in principle also catalyzes unwanted side reactions of the isocyanate groups (e.g., formation of allophanates, uretdiones, isocyanurates, etc.), the systems in question then no longer have sufficient shelf life.
Another disadvantage of the isocyanate-crosslinking adhesives is the health-related classification, which ranges from sensitizing to toxic. A critical factor here is the amount of monomeric isocyanates which remain, and which are removable only with difficulty. This presents problems for the end user, i.e., the craftworker or do-it-yourself user, who comes into contact not only with the fully cured and hence isocyanate-free and entirely unobjectionable product, but also with the isocyanate-containing adhesive, or monomeric isocyanates. For the unpracticed home improver there is a particular risk here that the products may not be used expertly and/or properly. Additional hazards arise here from incorrect storage as well, such as storage within the reach of children, for example. With the professional craftworker, on the other hand, proper use and storage can be assumed. Here, however, the problem exists that the professional user is required very regularly indeed—possibly even a number of times a day—to work with the isocyanate-containing material, something which is potentially critical in view in particular of the aforementioned sensitizing and also possibly carcinogenic effects of isocyanates.
Somewhat more favorable in this respect are isocyanate-crosslinking adhesives which contain only very low levels of volatile isocyanates and are therefore at least free from labeling requirements. These adhesives, however, are mostly based on aliphatic isocyanates, which in turn are less reactive. For applications where rapid setting of the adhesive is a factor, therefore, these adhesives are even more unfavorable than conventional PU adhesives.
An alternative curing technology which is finding application increasingly in the adhesives sector is that of silane crosslinking, where alkoxysilane-functional prepolymers, on contact with atmospheric moisture, initially undergo hydrolysis and then cure through a condensation reaction. The corresponding silane-functional—usually silane-terminated—prepolymers are entirely unobjectionable from a toxicological standpoint.
Polymer systems which possess reactive alkoxysilyl groups have been known for a long time. On contact with water or atmospheric moisture, these alkoxysilane-terminated polymers are capable even at room temperature of undergoing condensation with one another, with elimination of the alkoxy groups. Thus adhesives based on alkoxysilane-crosslinking polymers, in the fully cured state, exhibit not only good properties of adhesion to a number of substrates, but also very good mechanical properties, since they may possess not only tensile strength but also high elasticity.
Preference here is given in numerous applications to one-component systems (1K systems), which cure on contact with atmospheric moisture. The critical advantages of one-component systems include in particular their very great ease of application, since in this case there is no need for the user to mix a variety of adhesive components. In addition to the time/work saving and the reliable avoidance of possible metering errors, there is also no need with one-component systems to process the adhesive/sealant within a usually decidedly narrow time window, as is the case with multicomponent systems after mixing of the two components has taken place.
A disadvantage of these systems in line with the prior art is, in particular, the low reactivity of the corresponding MS polymers or SPUR polymers with respect to moisture, necessitating an aggressive catalysis. The mixtures in question therefore typically include considerable amounts of toxicologically objectionable tin catalysts.
An advantage here is the use of what is called α-silane-terminated prepolymers, which possess reactive alkoxysilyl groups joined via a methylene spacer to an adjacent urethane unit. This class of compound is highly reactive and requires neither tin catalysts nor strong acids or bases in order to achieve high cure rates on air contact. Commercially available α-silane-terminated prepolymers are GENIOSIL® STP-E10 or -E30 from Wacker-Chemie AG.
However, a disadvantage of the usual common silane-crosslinking systems is a relatively low tensile shear strength. Typical applications for this new type of adhesive are confined, consequently, in general to areas in which the requirement is for elastic adhesives more than for adhesives of high tensile strength.
In the case of which meet the European DIN EN 204 standard, durability class D4, silane-crosslinking adhesives are likewise achievable in principle—for example, with silane-terminated polyurethanes, of the kind described in WO 2011/026658. The high tensile shear strength here is achieved through the use of prepolymers which on the one hand have a very high density of urea units and/or urethane units that are capable of hydrogen bonding, and on the other hand are relatively short-chain and thus have a correspondingly high number of crosslinkable silane groups. Systems of this kind, however, inevitably possess two inherent disadvantages. First, the preparation of polymers with a high concentration of silane-crosslinking groups necessitates correspondingly large quantities of silane. These silanes, however, generally constitute the most cost-intensive prepolymer constituents, raising the raw materials costs for these products accordingly. Secondly, the high concentration of urethane groups and/or urea groups, which is likewise necessary for achieving high tensile shear strengths, leads to very high prepolymer viscosities. Corresponding problems affect both the compounding of these prepolymers to form fully formulated adhesives, and the application of these end products, which are usually likewise of comparatively high viscosity.